docs: def monthly_clear_sky_conditions(self):
          """A list of 12 monthly clear sky conditions that are used on the design days."""
          if self._monthly_tau_diffuse is [] or self._monthly_tau_beam is []:
              return [OriginalClearSkyCondition(i, 21) for i in xrange(1, 13)]
          return [RevisedClearSkyCondition(i, 21, x, y) for i, x, y in zip(
              list(xrange(1, 13)), self._monthly_tau_beam, self._monthly_tau_diffuse)]